Recently, wind turbine generators have been drawing attention as a source of clean energy that does not discharge greenhouse gases during generation. A wind turbine generator has wind turbine blades rotating around an axis by the force of wind and generates power by converting the rotational force to electric power.
The power generated by the wind turbine generator is a product of the shaft-end output (power generated by the blades) multiplied by the conversion efficiency (efficiency of the bearings and generator). The shaft-end output is given by the expression below. Power generation is increased by using blades with high blade efficiency and a large blade diameter.shaft-end output=½×air density×wind speed^3×blade efficiency×π×blade diameter/2^2
The blade efficiency has a theoretical upper limit (Betz limit=0.593). The actual upper limit is approximately 0.5 due to the influence of the wind turbine wake and air resistance of the blades. Thus, it is difficult to significantly improve the blade efficiency further.
Since power output increases as the square of the blade diameter, it is effective to increase the blade diameter in order to increase power generation. However, an increase in the blade diameter may cause an increase in aerodynamic load (the thrust acting in the inflow direction and the moment transmitted to the blade root). Such an increase may cause equipment such as the rotor head, the nacelle, and the tower to increase in size and weight, causing an increase in costs. Furthermore, an increase in the blade diameter may cause an increase in aerodynamic noise due to an increase in the tip speed at the blade tip. Hence, there is a need for an aerodynamics technology that achieves performance improvement and noise reduction without increasing the blade diameter.
Typically, a wind turbine blade has a predetermined optimal chord length corresponding to a certain tip speed ratio and satisfies the relationship represented by the following expression (Wind Energy Handbook, John Wiley & Sons, p. 378):Copt/R×λ2×CLdesign×r/R≈16/9×π/n  (1)where Copt is the optimal chord length, R (blade radius) is ½ of the blade diameter, λ is the design tip-speed ratio, CLdesign is the design lift coefficient, r is the radial position of a blade cross-section, and n is the number of blades.
The design tip-speed ratio is the ratio of blade-tip speed to infinite upstream wind speed. The design lift coefficient is a lift coefficient for the angle-of-attack corresponding to a maximum lift-to-drag ratio (lift/drag) of an airfoil (blade cross-section) and is determined in accordance with the (aerodynamic) shape of the airfoil (blade cross-section) and the inflow condition (Reynolds number).
FIG. 13 illustrates the relative wind speed at blade cross-section. As illustrated in FIG. 13, the Reynolds number of a wind turbine is determined in consideration of the relative wind speed at a cross-section A-A of a blade rotating at a predetermined rotating speed and is represented by the following equation:Reynolds number=air density×relative wind speed at blade cross-section×chord length of blade cross-section/viscosity coefficient of air
PTL 1 discloses an airfoil for increasing the wind-turbine power output. Specifically, it discloses an airfoil having a blade thickness ratio in the range of 14% to 45% and a design lift coefficient in the range of 1.10 to 1.25 (refer to Claim 1).